Faulty water temp guage reading?

Hi all any help gratefully received -
Our recently purchased Series 3 SWB has developed a problem!
It has a petrol engine which was put in around 2006. It still has the diesel dashboard. Have fitted a new temperature sender unit and it still reads too hot even when just started up, although there does not seem to be any overheating. Was wondering if the guage is different on a diesel and so causing the funny reading? PS Fuel guage is reading fine.
Thanks for any help

I think theres a ballast resistor , possibly behind the temp gauge in the dash . My bet is it's fubar .

Thanx!

We found the problem! Seller said it was the choke sender unit which was actually water temp unit!!! Out water temp sender unit just had a bolt!!! now sorted thanks for all help THANX